This invention relates to a method of setting up apparatus for handling electrical or electronic components, for example in a component placement machine. The components may be of a variety of shapes and sizes, for example so-called flat packs, S.O style transistors, leadless chip carriers, dual in line packages, melf type components and the like, as well as so-called "chips".
In the manufacture of electrical or electronic apparatus it is necessary to assemble a plurality of components on a suitable substrate, for example a printed circuit board. A number of systems have been proposed for handling the components to be placed on a substrate and many of these have proposed the use of pick-up heads having a suitable tool by which a component may be picked up. The tools have been of various types, depending to some extent on the components to be handled; for example the tools may grip the components mechanically or may use suction or a magnetic system to retain a component on the tool of the pick-up head when the component is removed from a suitable component supply means for delivery elsewhere, for example to a suitable placement position where the component may be placed on a substrate e.g. a printed circuit board. It is important to ensure that the components are correctly oriented when placed on the substrate and a number of methods have been proposed to attempt to ensure correct orientation.
In component placement machines of the type described, the substrate, for example a printed circuit board, on which the components are to be placed, is mounted in an accurately known position on an X-Y table, suitably mounted on a frame. The X-Y table may be mounted and driven by any suitable means, a number of which are known, and one such means for mounting and driving an X-Y table is described by way of example in our PCT patent application Publication No. W.O. 85/03404.
Whilst the movement of the X-Y table can in theory be determined accurately and precisely by control of its drive mechanism, there can in practice be some deviation between the theoretical and the actual response of the table to the drive mechanism. This can result in a deviation from the intended location of the X-Y table, and hence the substrate mounted on it, which can lead to inaccurate placement of a component on the substrate.
Clearly if this inaccuracy in placement of the component exceeds any permissible tolerance, then an unacceptably high reject rate for the completed substrates will result.
G.B. patent Specification No. 1 142 774 has addressed the problem of deviations in X-Y response in relation to numerically controlled devices such as plotters, machine tools or the like. The method described in that patent specification involves driving a part to a number of different positions relative to a reference member, detecting the true position of the part using a measuring device such as a laser interferometer, and storing a table of error values for future use in correcting table drive. This method requires the presence of a very accurate measuring device for measuring the parts position relative to the origin of the X-Y table coordinate system whenever the X-Y table is calibrated.
PCT patent Specification No. WO 85/03368 describes a calibrating system in which a plate with calibration marks thereon is mounted on a table which is movable in the X direction beneath a head which is movable in the Y direction. The x drive is utilized to drive to an x value which should correspond to a calibration point. If the calibration point is not located under a calibration probe used to provide a reference point, the x drive is utilized to move the plate by small amounts until the point is correctly located. The procedure is then repeated for y values using the same or a different, but nearby calibration point. This system is not concerned with an X-Y table and is time consuming since numerous attempts may be necessary to detect the calibration point in the x direction, as the calibration probe is only able to detect whether or not the point is correctly located, and then further attempts may be necessary for the y direction.